Move to ceilometer as CloudWatch metric/alarm back-end

Move to using ceilometer as the metric/alarm back-end for our Cloudwatch resources - this will require several in-progress new features in ceilometer, and rework in heat to make our current metric logic pluggable so it can be optionally replaced by ceilometer. This blueprint will be used to track which ceilometer features we require to proceed with this work, and to capture any design discussions around the heat-side work which is required.